It has been well known in the prior art to utilize solid state circuit assemblies for monitoring the presence of a fluid in a conduit or vessel. For example, such fluid presence sensors may be used in multiples as fluid level detectors in large fluid vessels, or may be used singly in flow conduits to protect machinery, for example, which would be damaged if fluid were not present in the conduit.
Prior art devices exist for sensing the presence or absence of a fluid, for example, water in the cavity, typically by means of a pair of electrodes exposed to the fluid. These devices generally consist of a solid state circuit for monitoring the resistance of the path between the two electrodes and thereby determining, through a reduction in this resistance path, that the monitored fluid is present. Typically, the electrodes in such devices are connected to a high gain, solid state amplifier which, in turn, drives a high current switching device used for operating equipment in response to the presence or absence of fluid. For example, if such detectors are used as level sensors in a fluid reservoir, the absence of fluid at a given level in the reservoir may trigger a pump to pump fluid to the reservoir. Alternatively, the presence of fluid at a higher level in the reservoir may be used to activate a pump to withdraw fluid from the reservoir.
In the prior art it has been common to utilize a relay as the high current switching device in order to avoid temperature problems within the switching member. Alternatively, solid state devices have been used in some prior art installations, in which case the high current switching transistor or other high current switching device generally requires a substantial heat sinc mounted outside of the fluid cavity or conduit for cooling the solid state switching device. The requirement for a relay, of course, substantially increases the size of the switching element as well as its cost, while at the same time reducing its long term reliability. On the other hand, the requirement of alternate prior art devices for heat sinc members, along with possible air cooling fins and, in some instances, fans for cooling these fins, substantially increases the complexity and cost of the monitoring system while at the same time increasing the labor costs involved in the installation of the sensor.